device for accurately measuring the mass of 240Pu and 239Pu in a sample having arbitrary moderation and mixed with various contaminants. The device utilizes a thermal neutron well counter which has two concentric rings of neutron detectors separated by a moderating material surrounding the well. Neutron spectroscopic information derived by the two rings of detectors is used to measure the quantity of 239Pu and 240Pu in device which corrects for background radiation, deadtime losses of the detector and electronics and various other constants of the system.
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